Ignore external SIGURG
Ignore external SIGURG

Go 1.14+ sends SIGURG to Ms to attempt asynchronous preemption of a G. Since it
can't guarantee that a SIGURG is only related to preemption, it continues to
forward them to signal.Notify (see runtime.sighandler).

We should ignore these signals, as applications shouldn't receive them. Note
that this means that truly external SIGURG can no longer be sent to the
application (as with SIGCHLD).

PiperOrigin-RevId: 291415357

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor